Microelectronic components, such as integrated circuits, semiconductors, and the like, which are termed workpieces in the present context, typically have many electrically conductive pads requiring conductive connection with one or more cooperative electrically conductive pads on the same workpiece or on one or more other associated workpieces. Conductive connection of the pads is conventionally provided by a wire segment which is bonded to a bond site on each of the pads using an automated wire bonding machine having a high degree of speed and precision. U.S. Pat. No. 6,102,275, which is incorporated herein by reference, discloses a bond head for a wire bonding machine and an associated automated high-speed wire bonding process which are used to produce two specific types of wire bonds in series termed ball bonds and stitch bonds, respectively.
In a simple case of the ball and stitch bonding process disclosed in U.S. Pat. No. 6,102,275, construction of an electrical connection between a first and a second bond site on a pad pair is initiated by feeding a wire from a wire supply to a capillary tool mounted to the bond head of the wire bonding machine. A free end of the wire, termed a wire tail, is suspended in air from the capillary tool and heated to form an essentially spherical, free air ball from the wire tail. The tool positions the free air ball at the first bond site, which is on one pad of the pad pair where a ball bond is desired. The tool applies a downward compression force to press the free air ball against the pad while transmitting bonding energy to the free air ball from a cooperative ultrasonic transducer likewise mounted to the bond head of the wire bonding machine. The combination of compression force and bonding energy deforms the free air ball onto the pad from its previously undeformed essentially spherical shape to a deformed more flattened shape, thereby joining the ball and pad at the first bond site to create the ball bond defined by the flattened ball.
Upon completion of the ball bond, the tool is displaced away from the first bond site along a predetermined pathway to a second bond site at the remaining pad of the pad pair which is on the same or a different workpiece. One end of the wire is retained in attachment with the ball bond at the first bond site while additional wire is played out to the tool from the wire supply. As a result, a wire segment extends from the ball bond at the first bond site to the tool at the second bond site. The tool applies a downward compression force to press the wire segment against the pad at the second bond site while transmitting bonding energy to the wire from the ultrasonic transducer. The combination of compression force and bonding energy deforms the wire against the pad from its previously undeformed intact shape to a deformed crimped shape, thereby joining the wire and pad at the second bond site to create the stitch bond defined by the wire crimp.
Upon completion of the stitch bond, the bond tool is displaced to a remote position away from the second bond site while the wire is retained in attachment with the stitch bond at the second bond site and additional wire is played out to the tool from the wire supply to form a new wire tail extending from the stitch bond at the second bond site to the tool at the remote position. The ball and stitch bonding cycle is completed by breaking off the wire tail from the stitch bond at the second bond site while retaining the stitch bond in engagement with the pad. The above-recited cycle may be repeated as often as desired on additional pad pairs.
Ball and stitch bonding is also applicable to more complex cases where a serial electrical connection is desired between a series of three or more pads. In accordance with the more complex case, a ball bond is formed on the first pad of the series in the manner recited above and stitch bonds are subsequently formed on each succeeding pad in the series. However, the wire is not broken off from each stitch bond as in the earlier case, but is only broken off when the final stitch bond in the series is completed.
An alternate related microelectronic application of the bond head of U.S. Pat. No. 6,102,275 is disclosed in U.S. Pat. No. 6,622,903, which is also incorporated herein by reference. In accordance with U.S. Pat. No. 6,622,903, the bond head of the wire bonding machine is employed to mount a ball bump to a bond site on a surface of a workpiece. The ball bump has subsequent utility for conductively bonding the workpiece on which the ball bump is mounted to a bond site on the surface of another workpiece. Alternatively or additionally the ball bump has utility for controlling the geometric spacing between the bonded workpieces or the joint height of the bonded workpieces.
The workpieces are usually either a wafer or a substrate. A wafer typically consists of a plurality of grouped die which all share a continuous common surface. A substrate is typically a relatively large planar structure such as a printed circuit or an integrated circuit package. In an exemplary microelectronic application of a ball bump, each die on a wafer is a tiny semiconductor component, such as a diode, transistor or integrated circuit, which has one or more die pads on the surface of the die, each die pad defining a discrete bond site. A ball bump is mounted to each die pad on the wafer and the wafer is subsequently cut into individual die, each of which has one or more ball bumps mounted thereto depending on the number of die pads on the surface of the die. Thereafter, each die pad is bonded to a corresponding bond site on the surface of a substrate by means of the ball bump on the die pad, wherein each bond site on the substrate is defined by a substrate pad.
The ball bumps are mounted to the die pads on the wafer using substantially the same automated high speed wire bonding process described above to form ball bonds. However, in the case of ball bumps, formation of the stitch bond is omitted after each free air ball is deformed and correspondingly mounted to a die pad on the wafer. Instead, the wire is broken away from the ball bump after deformation and mounting. The resulting ball bumps, nevertheless, have essentially the same deformed flattened shape relative to the undeformed free air ball as do the ball bonds. Likewise, the ball bumps join the die pads with the substrate pads in a manner similar to ball bonding by compressing all the ball bumps against the pad substrates in unison while simultaneously heating or otherwise applying energy to the ball bumps. Because the ball bumps are formed from an electrically conductive metal, the ball bumps function not only as an adhesive joint, but as an electrical conductor between the die and substrate.
Yet another alternate related microelectronic application of a bonding machine, which has a bond head and tool mounted thereto, employs the bond head to electrically connect a tab lead to a die or substrate pad. The tab lead is one of a plurality of tab leads, which are electrically conductive and which have been press mounted into a pattern on a flexible medium. The bond head bonds the tab lead to the pad at a bond site, which results in an electrical connection between the tab lead and the pad termed a tab bond.
The structural integrity and operational performance of ball bumps, ball bonds, stitch bonds, tab bonds, and other like deformed bond members, which are produced in the manner described above, are inter alia a function of certain geometric parameters of the deformed bond member, such as the height of the deformed ball bond or ball bump for a given free air ball diameter, the impression depth of a stitch bond for a given wire diameter, or the impression depth of a tab bond for a given tab lead thickness. It has specifically been found that the degree of deformation the bond head imposes on an undeformed ball, wire, or tab lead is a good predictor of the ultimate integrity and performance characteristics of the deformed bond member. Accordingly, the operator effects process control for production of the deformed bonded structure by 1) predetermining a fixed bonding energy application time which the practitioner believes will achieve a desired degree of deformation of the final product, and 2) coupling the bonding energy from the activated ultrasonic transducer to the undeformed free air ball, intact wire, or intact tab lead for the fixed bonding energy application time to produce a deformed bond member.
Although the above-recited process control procedure results in a relatively high percentage of acceptable final products, significant variability nevertheless remains in the geometric parameters of the ball bonds, stitch bonds, ball bumps, and tab bonds over a large sample of such deformed bond members. This variability typically follows a normalized distribution and produces a significant number of unacceptable final products, i.e., ball bonds, stitch bonds, ball bumps, and tab bonds, exhibiting values of geometric parameters outside acceptable limits. The acceptable limits for the values of geometric parameters have become increasingly finer as the microelectronic components and the electronic systems in which the components are utilized have in general shrunken in size.
For any given geometric parameter, the variability of the parameter is the result of many inherent process variables which influence the coupling of bonding energy from the ultrasonic transducer to the free air ball, intact wire, or intact tab lead. Such inherent process variables include the composition of the material from which the pad, wire and/or tab lead are fabricated, the configuration and dimensions of the bond site and the wire, and the degree of wear exhibited by the tool, to name but a few. Unfortunately many of these inherent process variables are neither readily measurable nor readily controllable by the operator of the bonding machine. Accordingly, the practitioner must accept deformed bond members exhibiting geometric parameters which have an undesirably broad distribution as an unavoidable consequence of conventional automated high-speed bonding processes.
The present invention recognizes a need for an automated high-speed bonding process which produces ball bonds, stitch bonds, ball bumps, tab bonds, or the like with geometric parameters having a relatively narrow distribution, such that the geometric parameters fall within acceptable production tolerances. Accordingly, it is generally an object of the present invention to provide an automated high-speed bonding process which produces ball bonds, stitch bonds, ball bumps, tab bonds, or the like with geometric parameters having a relatively narrow distribution. More particularly, it is an object of the present invention to provide a process control procedure for operation of an automated high-speed bonding machine which enables the machine to produce ball bonds, stitch bonds, ball bumps, tab bonds, or the like with geometric parameters having a relatively narrow distribution. It is another object of the present invention to provide a process control procedure adapted to operation of a conventional bonding machine which enables the machine to produce ball bonds, stitch bonds, ball bumps, tab bonds, or the like with geometric parameters having a relatively narrow distribution. These objects and others are accomplished in accordance with the invention described hereafter.